Bridge building was also practiced in the epoch of primitive society. The prototype of a bridge could be a log thrown over a stream or a ravine, or suspension bridges, woven from ropes, with light planking suspended from them and fixed on the opposite sides of a canyon. These structures helped man to overcome obstacles on his way. The ancient Oriental and ancient slave-holding states built better wooden and stone bridges. We know about the construction of wooden bridge Siblicius in Rome (630 B.C.), arch bridge over the Danube consisting of 21 spans of 36 m each (Troyan Bridge, 103 A.D.). The spans were covered with wooden arches and rested on tall concrete piers lined with stone.
Among the most remarkable constructions in Roman architecture are the arcades of aqueducts, one of which is the bridge of Garda (II c. A.D.). The Garde Aqueduct crosses the deep valley of the River Gard near the city of Nîmes in France. The lower tier of the arcade, 21.6 m high and 6.36 m wide, consists of six arches and carries a second tier, 21.5 m high and 4.56 m wide. The third tier has a height of 7.82 m and a width of 3.06 m, and is fed by an aqueduct. In this level there are 35 small arches. The total height of the structure is 48.77 meters. The length of the aqueduct is 275 metres along the top, 242 metres along the second tier and 142 metres along the lower arcade. The arches of the first tier and the arches of the second tier above them coincide vertically. The number of small arches of the third tier above the middle arch, wider than the others, is four, and three each above the others.
The combination of the spans and piers in the complex tier system, designed in arched forms and with clearly defined proportions, was conducive to the creation of a perfect artistic composition of the bridge.
The aqueduct also served as a bridge for horseback riding, thanks to the 90 centimetre narrower second tier on the ledge. In the 14th century, the lower part of the second tier uprights was undercut and the distance to the ledge was widened by another 1.5 metres, and the cross-section of the uprights was weakened by 1/3 of its cross-section. The bridge stayed like that for another 400 years. In XVIII century it was restored and a bridge was added to the lower stage, having precisely repeated the outlines of the arched spans, piers and the height of the second stage.
Roman architects and builders used semi-circular vaults and pozzolanic concrete in constructing the bridges, and constructed tongue and groove railings and pile foundations. To reduce the construction time, a modular system of elements was used: supports of the same width, which were 1/2-1/3 of the span’s size, were repeating spans.
The general shape of the bridges, built by Romans, was notable for its individuality and harmoniously blended in with the environment, in what ancient architects had reached high art.
In the next centuries during the construction of bridges along with stone they began to apply high-strength brick and more gentle arches, the sizes of spans overlapped increased and more perfect outlines of the bridges were found, which contributed to the change of their shape. However, Roman bridges and aqueducts will forever remain unsurpassed monumental structures of the time (Fig. 5.3).
Historical monuments testify about achievement of high level of technique in processing stone for erection of cult buildings, construction of roads, bridges and tunnels by ancient civilizations of Incas and Aztecs in VI-VII centuries in South America.
Around 1350 in the Inca state a suspension bridge with a span of 90 meters was built over the Apurimac river gorge. The base of the structure was ropes with a diameter of 90-100 cm, woven from the fibers of American agave. At the approach to the bridge a 100-meter tunnel was built in the rocks. The bridge was in use for 540 years and was not replaced until 1890.
Hanging bridges, similar in design, were also built in China, in some parts of Central Asia and Transcaucasia.
After the collapse of the Roman Empire in V century A.D. the development of bridge-building slowed down for a few centuries. It was the period of the Middle Ages. In the first half of the Middle Ages, in the epoch of feudalism, there was no evident progress in the field of bridge building. They continued to build massive stone bridges and partly wooden ones on piles. Bridges were also built in ancient Byzantium. Bridges had lancet or more gentle circular arches.
In the VI-VII centuries, trade routes moved to Italy and France. During this period, there is a need to build new bridges and reconstruct old ones on the routes for the movement of goods. Especially many bridges are built in France and later in England. Large bridge crossings were erected by special brigades, the so-called “bridge brothers”, established at the monasteries. During this period, outstanding bridges in architecture and size were built in the south of France. The bridge over the Rhone at Avignan (1178-1187). The bridge builders used circular masonry vaults in the tradition of ancient Rome.
One of the most beautiful bridges of that time was the bridge-fortress built in the mid-13th century across the Lot River in Cahors. The bridge was guarded by three combat towers. The architecture of the bridge was based on the alternation of arched arches on high piers and watchtowers reflected in the river water. The most famous is the Ségué Bridge, built in France in 1336. It is a single-span bridge with a semi-circular vault. The bridge has a span of 45.45 m, the thickness of the vault in the lock is 1.3 m, and the width of the vault is 4.0 m. The width of the carriageway on the top of the bridge was 2.5 m, with sidewalks on both sides. Above the large vault, vaulted openings of 8.0 m span were arranged. The vault of the bridge was built of roughly hewn stones. In 1741, the bridge was repaired and reconstructed by reducing the steepness of the entrances.
In Italy in 1356 the bridge in Verona is erected, which has three spans with arches of low (1:4) circular shape. The largest span is 48.7 m, the width of the deck on the bridge is 6.5 m. The lower part of the bulls is faced with red and white marble. The marble blocks were 0.56 m thick, and 2.0 m long. Above everything was made of brick, only the sides of the vaults were lined with limestone. But most interesting is the architecture of the bridge in the form of a fence with dovetail ends. The appearance of the bridge with loopholes and towers has a typical medieval military appearance.
In the 1370s the Vieille Brioude Bridge was built. The vault of the bridge was circular and spanned a span of 54.26m, with a gib of 18.8m. The bridge was built of volcanic stone on a lime mortar. This bridge had the highest span until it collapsed in 1822.
The desire to increase the spans of bridges was due to the difficulty of laying the foundations for supports in the rivers. More than one generation of bridge-builders worked out technical methods of making foundations for the supports during XIII-XIX centuries.
An example of a solid foundation for supports is the bridge 81. – The bridge was built in 1265-1307. The thickness of the piers was 1: 3 spans. The foundations for the abutments are very wide, presumably on a wooden plinth, built on a stone scaffolding.
The use of small spans and shallow arches avoided steep entrances to the bridge and made it possible to build city stone bridges. Urban bridges not only served the function of crossing various obstacles, they also accommodated dwellings and commercial houses, in which case the bridge bed was of greater width. The width of Notre-Dame Bridge in Paris built in 1507 was 23.6 m. The bridge had semi-circular arches and small spans of 16-17 m.
In the 16th-17th centuries, the development of wheeled transport influenced a further decrease in the steepness of entries on bridges, especially city bridges. Vaults became gently box-shaped and were widespread in the XVIII century. The thickness of the beams decreased to 1:5 of the span, the foundations under the beams in the form of a stone scaffold were rejected. Pile foundations were placed 1.5-1.8 m below the water surface, and then 4.5 m.
The bridge in Toulouse took 90 years to build, from 1542 to 1632. It had seven asymmetrically placed spans of 14.4 to 34.4 m. The spans were covered by box vaults resting on stone piers. The bridge deck was supported by brick walls with ornamental holes above the piers. The slope of the bridge deck was more than 4%.